The present invention relates to an apparatus for conveying and folding a flexible packaging material, comprising a conveyor with conveyor devices which, along a section of the length of the conveyor, have gradually modified orientation.
In the production of packaging containers of various types, e.g. for packing liquid foods such as milk, juice or the like, use is often made of laminated material, a so-called packaging laminate consisting of layers of different material types. A common laminate material thus comprises a relatively thick core or carrier layer of fibre material, e.g. paper, which is coated on either side with thin layers of liquid-tight plastic material. The plastic material is normally of the thermoplastic type, i.e. a plastic type which can be sealed by heating and compression, which is utilised to seal the packaging material in connection with its conversion into packaging container blanks or finished packaging containers. The thermoplastic may, for example, be polyethylene or polypropylene. When necessary, the packaging laminate may also contain additional layers of, e.g. barrier material such as aluminium foil (Alifoil) or a per se known barrier plastic.
As a preparatory measure to the conversion of the packaging laminate into finished, filled and sealed packaging containers, a conversion often takes place of the original web-shaped packaging laminate into individual sheets or blanks in a special prefabrication operation. A typical blank for producing, e.g. gable top packages, consists, for instance, of a separate packaging laminate portion which, in a per se known manner, has been provided with a pattern of weakening or folding lines (crease lines) in order to make possible folding and reforming into a finished, gable top-fitted packaging container. In addition to the crease lines which are necessary to make for reforming of the top and bottom of the packaging container into finished form, the blank is provided with a number of parallel crease lines so as to convert the blank into sleeve or tube form of substantially rectangular cross section. This takes place in a prefabrication operation which comprises conveying the packaging container blank in the longitudinal direction of these crease lines and progressive folding of the outer panels defined by the crease lines so that these are each folded through 180xc2x0 and with their outer, free side edges slightly overlapping each other. By heating the external thermoplastic material, a fusion or sealing thereafter takes place in this overlap zone which, after compression, forms a liquid-tight, longitudinal sealing joint or seam along the entire length of the packaging container blank. This pre-processing of the packaging laminate into individual, tubular and flat-laid packaging container blanks takes place at relatively high speed, at present typically of between 100 and 200 m/min. If the intention to further increase this production output speed, certain difficulties will arise as regards handling of the blanks, since conveying the blanks at even higher speed will result in instability which makes it difficult to obtain a uniform heating result satisfactory for the subsequent sealing operation.
At extremely high conveyor speeds, e.g. up towards 300 m/min, a rapid and intensive heating of the thermoplastic region which is to serve as the sealing zone is also necessary. In such instance, the temperature on heating must either be increased considerably, or the heating distance must be extended. At the same time, in order to obtain an acceptable degree of efficiency, a short distance is necessary between the heating device, e.g. a hot air nozzle, and the surface of the packaging material, which in turn necessitates extremely accurate guiding of the outer parts of the packaging container blank which, after folding, are to be sealed to one another. The method hitherto employed of heating the outer, longitudinal edge regions of the sheet before folding of the sheet has been commenced at all moreover proves to entail that the heated thermoplastic, during the folding phase, will have time to be cooled to such an extent that the subsequent seal will be of defective quality. This can partly be avoided in that the two panels which are to be heated are first folded to a 90xc2x0 angle with the remaining panels and thereafter heated, whereafter the remaining 90xc2x0 folding is finally undertaken and the overlapping panels are compressed together and sealed. With this technology, the distance and time between heating and sealing will be reduced, which increases the possibility for a fully adequate seal even at high conveyor speeds. However, this stepwise method further renders the guiding of the panels more difficult in connection with the heating operation, and trials employing support rollers and the like have merely resulted in the edges of the blank which are to be heated having an undulating appearance, with consequential uneven heating. Since, because of the inherent flexibility of the panels, neither is it possible to reliably be able to guide them in the immediate vicinity of the discharge surface of the hot air nozzle, heat losses will be great at the same time as the heated region will have an uneven and undefined contour.
Thus, there is a general need in the packaging or conversion technology to realise an apparatus for conveying and folding flexible packaging material which makes it possible, in connection with the folding at high speed, to effectively guide and heat the longitudinal free edge regions of a blank in such a manner that, with minimum losses, a uniform and well-defined heating region is obtained which, after supplementary folding together of the edge regions of the blank may be utilised for thermosealing of the overlapping edge regions, for the formation of a liquid- and possibly gas-tight seal extending along the blank.
One object of the present invention is to realise an apparatus for conveying and folding flexible packaging material, the apparatus making it possible, in a reliable and well-defined manner, to heat a longitudinal edge region of a packaging container blank at the same time as this is conveyed with high precision and at high speed, and is folded and sealed for the formation of a longitudinal seal.
A further object of the present invention is to realise an apparatus of the above-mentioned type, the apparatus being capable of conveying the blank in such a manner that the edge region intended for heating is guided rectilinearly along a well-defined path adjacent a heating device.
Yet a further object of the present invention is to realise an apparatus of the above-mentioned type, the apparatus making it possible, with minimum thermal or friction losses, to reliably guide and heat the edge region of the blank.
Still a further object of the present invention is to realise an apparatus of the above-mentioned type, the apparatus making it possible to guide the blank in such a manner that the edge of the blank will, to the greatest possible extent, remain planar and freely accessible for heating, folding or other processing.
Yet a further object of the present invention is to realise an apparatus of the above-mentioned type, the apparatus being particularly suitable for high production speed, being simple to manufacture and maintain and not suffering from the drawbacks which are inherent in prior art constructions.
Solution
The above and other objects have been attained according to the present invention in that an apparatus of the type described by way of introduction has been given the characterizing features as set forth in the characterizing clause of the appended main claim.
Preferred embodiments of the apparatus according to the present invention have further been given the characterizing features as set forth in the appended subclaims.
Advantages
By providing the apparatus according to the present invention with a perforated belt and a partial vacuum device which extends substantially along all of that part of the belt which runs with unchanged orientation immediately adjacent the hot air nozzle, it will be possible, on the one hand, to increase the precision in the guiding of the edge region of the blank, and, on the other hand, to ensure that the blank remains planar so that the edge region can thereby be led in immediate conjunction with the nozzle. As a result, thermal losses are reduced at the same time as a well-defined heating zone is obtained. In that the holes have different surface areas on both sides of the belt, maximum suction effect on the blank will be obtained at the same time as the suction effect does not create unnecessarily large friction between the rear side of the belt and the work surface of the suction box. It is also possible to provide the suction box with a coolant duct which further reduces heating of the belt. The apparatus according to the present invention thus hereby makes it possible, at high speed and with great precision, to realise a satisfactory and well-defined heating of the longitudinal edges of packaging container blanks which are to be thermosealed in a longitudinal sealing joint.